Various displacement sensors are known that provide signals which are indicative of a position using a linear variable differential transformer (LVDT). This LVDT type of system may be regarded as capable of precise or ultra-precise measurement (e.g., precision at the level of one micron, or much better), particularly over limited measuring ranges (e.g., 10 mm, or 5 mm, or less). An LVDT may use a primary solenoidal coil which generates an induced current in two secondary solenoidal coils, and the magnitude of the current in each secondary coil depends on the position of a ferromagnetic core that moves axially inside the coils. When the secondary coils are connected in reverse series, the position of the ferromagnetic core may be determined based on a resulting differential signal.
In some applications, it is desirable to provide an optical displacement sensor which provides a linear displacement signal, in a precise manner analogous to that of LVDT-type sensors. One prior art system of this type is disclosed in U.S. Pat. No. 4,338,722 (the '722 patent) issued to Delmas, which is hereby incorporated herein by reference in its entirety. The '722 patent discloses a sensor with a light source, a detector comprising two photoreceiver detectors and a moving stem with an opening to transmit light from the light source to the detectors. The moving stem slides between two guides. A cover over the opening on the moving stem is designed to be substantially symmetrical with the contact tip at the stem end relative to the center point between the two guides, as a means of eliminating measurement sensitivity to mechanical play of the stem. However, the '722 patent fails to recognize certain errors that may be associated with the light source. Thus, the device of the '722 patent may suffer from undesirable signal nonlinearity and/or accuracy errors.
A superior light source configuration is disclosed in U.S. Pat. No. 4,812,635 (the '635 patent), issued to Kaufmann et al., which provides a means for homogenous illumination in a position sensing device. The device of the '635 patent includes a light source, two photodiodes and a moving diaphragm which contains an aperture. The two photodiodes produce a signal determined by the position of the light which passes to them from the light source through the aperture on the diaphragm. The means for homogenous illumination comprises a correction filter along the optical path, which in the preferred embodiment comprises a film negative exposed using the light source at its nominal spacing. Such a custom filter outputs illumination which nominally has a uniform intensity. However, although the '635 patent provides source light which is uniform in intensity along a measuring axis, certain other errors that may be associated with the light source remain unrecognized. Thus, like the '635 patent, the device of the '722 patent may also suffer from undesirable signal nonlinearity and/or accuracy errors.
It would be desirable to provide a precision optical position sensing device that is a simpler and lower-cost device than those outlined above, and which provides improved linearity and/or accuracy (e.g., relative to the devices of the '635 and '722 patents).